One picture covers the whole word: something beaten until it is full of tiny bubbles, so that it turns soft, light and able to hold a shape without being solid. The word came straight from French, where it means foam. Chocolate in a glass, salmon on toast, foam from a can worked into damp hair — the material changes every time, the air inside never does. If you can feel that lightness, you can tell which kind is meant just from what stands next to it.
From the word’s history
From French mousse ('foam, froth'), from Old French mosse ('moss'), from a Germanic word for moss and bog. The foam is not a later image laid on the word; it is what the word meant when English borrowed it.
來自法文 mousse（泡沫），再往前是古法文 mosse（苔），源頭是日耳曼語系表示苔、濕地的字。所以「泡沫」不是後來加上去的想像，而是英文借這個字進來的時候，它本來就有的意思。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1113414260:12851), 'mousse' etymology
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